This invention relates to a method and apparatus for phase shift keying (PSK) modulation using digital echo modulation techniques.
In PSK modulation, a sine wave carrier signal of fixed frequency is used. Binary data values are encoded in changes in the phase of the carrier signal between signaling intervals. For differential phase shift keying (DPSK), the reference point from which the phase angle is measured at any signaling interval is the phase angle of the immediately previous interval. Thus, with DPSK the signal is self-referenced so that no separate absolute phase information needs to be transmitted. The DPSK encoding technique used in the Bell 212 modem encodes two bits (called a dibit) into one signaling element (baud). The values used are as follows:
______________________________________ Phase Change Dibit (Degrees) ______________________________________ 00 90.degree. 01 0.degree. 10 180.degree. 11 270.degree. ______________________________________
When PSK or DPSK is used, the change in phase between signaling elements can result in abrupt transitions in the signal, such as a 180.degree. instantaneous change from a maximum positive value to a maximum negative value. This instantaneous change essentially makes the carrier signal a very high frequency signal for that short period of time. This high frequency component can cause problems for several reasons. For instance, in a typical telephone line frequencies above about 3000 Hz are attenuated by the impedance of the telephone line, and thus the usable bandwidth is only about 3000 Hz. Thus, high frequency components are lost in transmission and errors can result in the received signal. Alternately, two channels may be used for transmission, i.e., a 1200 Hz carrier signal for sending information in one direction and a 2400 Hz carrier signal for sending information in the other direction. High frequency components of the modulated 1200 Hz carrier signal could interfere with the 2400 Hz signal.
FIG. 1 shows an example of two such channels in the 3000 Hz usable voice grade telephone channel. A first channel (band) 10 for transmission in one direction is centered at 1200 Hz and extends from approximately 600 Hz to 1800 Hz with a total bandwidth of 1200 Hz. A second channel 12 for transmission in the opposite direction is centered at 2400 Hz and extends from approximately 1800 Hz to 3000 Hz. It can be seen that if a high frequency component generated by modulating the 1200 Hz carrier falls within upper channel 12, interference will result. One technique used to reduce the high frequency components introduced by PSK is called digital echo modulation.
Digital echo modulation is described in a paper by Alain Crosier and Jean-Marc D. Pierret entitled "The Digital Echo Modulation," I.E.E.E. Transactions On Communication Technology, p. 367 (August, 1970). The technique for digitally implementing digital echo modulation is set forth in "Microcoded Modem Transmitters," M. F. Choquet and H. J. Nussbaumer, I.B.M. J. Res. Develop., p. 338 (July, 1974). A brief, simplified description of digital echo modulation as described in these references is set forth in the following paragraphs.
Basically, digital echo modulation is a technique used to produce a modulated carrier signal with few unwanted frequency components outside the transmission bandwidth. This is done by a combination of two interrelated techniques. First, a signal element representing a dibit is shaped so that substantially all of its frequency components are in the desired transmission band. Second, a number of signal elements are overlapped to smooth out transitions between signal elements.
The shaping of the signal element is done using Nyquist's telegraph theory. This shaping involves using a mathematical formula to form a composite signal from a series of frequencies in the desired frequency band. This composite signal is the Nyquist-type time-domain signal element shown in FIG. 1B. A 1200 Hertz signal 14 is shown within the Nyquist envelope 16. In addition to a primary component 18 of the signal element there are a number of echoes 20. The purpose of the echoes is to cancel the undesired frequency components of the primary component 18. The amplitude of the signal element trails off infinitely in both directions.
FIG. 1C shows the signal element of FIG. 1B with all but the primary echoes eliminated. The signal element of FIG. 1C is also modified so that when transferred back into the frequency domain, one of the original frequency bands of FIG. 1A will be closely approximated. The formula used to modify the signal element so that the element and only its first echoes will most closely give the desired frequency band is called a "window function."
Digital echo modulation involves digital generation of the signal element of FIG. 1C. Information is coded in these signal elements by changing the phase of the signal from one element to the next. However, this will result in abrupt changes between signal elements, introducing high frequency components into the transmitted signal. This undesirable feature is eliminated by using an overlapping technique as shown in FIG. 2.
FIG. 2 shows four different signal elements 22, 24, 26, 28 representing a first through a fourth dibit, each extending for a period 4T. The four signal elements are overlapped by spacing each a period T from the preceding signal element. Each of the signal elements is represented digitally by taking a number of samples along its length. For example, signal element 22 may be represented by 64 digital samples. Each of the signal elements is digitally combined with the other signal elements to produce a composite signal 30. For instance, to produce a sample value 32 of composite signal 30, a negative value 34 of signal element 28 is combined with a positive value 36 of signal element 26, a negative value 38 of signal element 24 and a positive value 40 of signal element 22. Other points of composite signal 30 are similarly generated.
Composite signal 30 will have smooth transitions from one element to the next due to the overlapping effect. However, the overlapping does not reduce the integrity of the data since phase shift information is determined from the state of the signal at the center of each signal element. As can be seen from FIG. 2, at the point where the Nyquist envelope of each signal element peaks, the remaining overlapping signal elements all have a null. For instance, at the time indicated by dotted line 42, signal element 24 alone determines the state of composite signal 30 because the other three signal elements are at zero at this point. This will also be true for signal elements which are phase-shifted by a multiple of 90.degree.. Thus, only the desired signal element will be produced in the composite signal at that instant in time corresponding to the end of a period T.
FIG. 3 shows a modulator for implementation of digital echo modulation. The data is input to a shift register 42 and from there proceeds to a number of signal element memories 44. Each signal element memory stores a digital representation of each of the desired phases of the signal. If four phases are used for each of two channels and each signal element is represented by 64 samples of 6 bits each, four 6.times.512 memories are required. The outputs of memories 44 are provided to a summation circuit 46 which produces a composite signal. The composite signal is processed through digital to analog converter 48 to produce an output signal.
A divider 50 operates to produce addresses to cause memories 44 to produce a digital amplitude at each of the sample points for a given signal element. The signal element is designated by the data in shift register 18 which is input to two address lines of a memory 44. After divider 50 causes all the samples of a signal element to be produced, shift register 42 will shift the data up and the divider will then again run through the addresses to produce the samples of each signal element. Thus, at any one time four overlapping signal elements are summed.
For implementation in an integrated circuit it is desirable to reduce the amount of required circuitry for the modulator. In particular, it would be desirable to reduce the memory required, because the memory circuits occupy a large amount of space due to the large number of signal element representations required.